The present invention relates to a method and to a device for propelling a cable or the like into a duct. It is most particularly, although not exclusively, suitable for installing electrical or optical cables inside pipes already buried in the ground.
Methods and devices are already known, for example from documents U.S. Pat. No. 4,185,809, GB-A-2,171,218, U.S. Pat. No. 4,941,774, GB-A-2,229,549, EP-A-0,264,767, EP-A-0,427,354, EP-A-0467,463, EP-A-0,665,993, U.S. Pat. No. 5,762,321, WO 92/17927 and WO 99/40660, which make it possible to propel a cable into a duct by means of a pressurized fluid introduced into the latter and capable of pushing or pulling said cable, the cable possibly being provided with fins and/or being attached to a pulling head which forms a piston inside the duct. Moreover, the fluid may be a liquid, such as water, or a gas, such as air.
Owing to the fact that the technique of pneumatic propulsion using a pressurized gas is expensive and only allows cable to be installed over limited duct lengths, the technique of hydraulic propulsion using a flow of liquid is generally preferred. The latter technique allows cables to be installed over very long duct lengths, is relatively easy to use and is not expensive. These advantages stem especially from the fact that the cable floats on or in the flow of liquid, thereby reducing the friction between the cable and the internal wall of the duct and allowing the cable to be advanced gently inside said duct.
However, such propulsion using a flow of liquid requires large amounts of liquid at a high pressure whenever it is desired to make the cable advance rapidly in a very long duct. This is because it will be readily understood that the speed of advance of the cable in the duct depends on the flow rate of the liquid in the latter and that the pressure losses are greater the longer the duct along which the cable has to travel.
Moreover, propulsion using a flow of liquid has the additional drawback that the cable, which generally floats on the surface of said liquid, can come into contact with the upper wall of the duct, a situation which then generates frictional forces, slowing down the advance of the cable.
The object of the present invention is to remedy these drawbacks. It relates to a method and to a device for liquid propulsion, allowing cable to be rapidly propelled into the duct without requiring said liquid to have a high flow rate and a high pressure, while preventing friction between the cable and the upper wall of the duct.
For this purpose, according to the invention, the method for propelling a cable or the like into a duct by means of a flow of a pressurized liquid, both said cable and said liquid being introduced at one of the ends of said duct, is noteworthy in that a stream of pressurized gas is also introduced into said duct, at said end of the latter, and in that the pressure of said gas is greater than the pressure of said liquid.
Thus, the pressurized gas injected into the duct fills those parts of the latter which are left free by the flow of pressurized liquid and by the cable and helps the liquid to entrain said cable. In addition, the pressurized gas allows the frictional forces between the cable and the internal wall of the duct to be limited. It therefore makes it possible, for a given liquid flow rate, to advance the cable more rapidly and a greater distance.
If, as is usual, the pressure of the liquid is increased in accordance with the distance traveled by said cable inside said duct, in order to compensate for the corresponding pressure losses, arrangements are made to ensure, according to the present invention, that, for each value of the liquid pressure, the gas pressure is equal to said value of the liquid pressure increased by a constant pressure surplus.
Experience has shown that, by virtue of the present invention, the maximum value of the liquid pressure in order to obtain satisfactory propulsion was at most equal to 15 bar for a ratio of the cross-sectional area of the cable to the cross-sectional area of the duct (occupancy) of about 60%. The constant pressure surplus may then be about 0.5 bar.
The present invention also relates to a device for propelling a cable or the like inside a duct, comprising first means for introducing said cable at one of the ends of said duct and second means for introducing a flow of pressurized liquid at said end of the duct in order to propel said cable. According to the invention, this device is then noteworthy in that it includes third means for introducing a stream of pressurized gas into said duct, at said end of the latter.
Preferably, said third means for introducing the stream of pressurized gas lie upstream, with respect to the direction of advance of said cable, of said second means for introducing the flow of pressurized liquid. In this case, it is advantageous for the device to include a nonreturn sealing means placed between said second and third means, which nonreturn sealing means allows said stream of pressurized gas to flow through it but prevents said flow of pressurized liquid from being directed upstream.
Also preferably, said first means for introducing the cable lie upstream of said second and third means for introducing the pressurized liquid and the pressurized gas. It is then advantageous for said nonreturn sealing means to be of the hydraulic high-pressure labyrinth seal type, through which seal said cable passes. In one embodiment, said nonreturn sealing means may then comprise a conduit whose internal diameter is slightly greater than the external diameter of said cable so that the latter can pass through said conduit while being in loose contact therewith, the internal wall of said conduit being provided with a plurality of grooves having a diameter greater than the internal diameter of said conduit.